Bonnet Spacers... Do they actually work?

[alexwilliams840]

I've been looking into ways to get the under bonnet temperatures lower for turbo builds.

I know a lot of people use them as it's the easiest, cheapest way and it seems like it will work. But I've had other people say because of the high pressure area in front of the windscreen that raisers don't do jack for cooling under the bonnet.

Does anyone have any first hand experience of them actually providing cooling? I'm getting they do work from some people and they don't from others

Just thought I'd bring the topic to this forum to see what you guys think

 

[Cartman]

A friend of mine runs a 550ish hp Celica, and he has both raised the rear of the Hood, and made a scoop, to suck air in, i seem to remember him adding the scoop, to get more Cold air in, because the spacers at the rear wasnt enough, but then again, 550 hp

 

[alexwilliams840]

The spacers are the rear won't let cold air in, just let hot air out (in theory)

 

[Cartman]

I know, his Celica has a werry closed off front, as aposed to the puma, might work better. But like you said, the high pressure at the front screen might be an issue..

If you Space up the bonet, ams Fitt some tassels, you should be able to se the air flow, if they stay Down, then there is Your answwr

 

[alexwilliams840]

I was just looking at the profile of the Puma, it's pretty sleek so I think it might work with it. Good idea, I might try that out just for confirmation

 

[PumaNoob]

I really don't like the look of bonnet spacers, but with my rather "full" engine bay with the turbo, they definitely help. He theory is that heat rises, so when it exhales the hot air from the top, it should induce cold air from underneath. Ive actually set my engine bay on fire at the bulkhead when I had the bonnet without spacers or vents.

The only issue is that there is a shield at the rear for a reason, it stops the engine bay air and fumes blowing through your heaters, once you break that seal, its not as effective, its not been an issue for me but its worth bearing in mind.

 

[alexwilliams840]

Thanks mate. I was thinking about them for the turbo too. It's such a small area to have all that heat so I suppose anything is worth doing to keep the temps down

 

[PumaNoob]

Yeah its not a good idea to have the turbo tucked down in that confined space with no air circulating and your hydraulic clutch and brake lines in there as well as fuel lines! You have the power steering lines on the 1.7 too, so any extra air circulation has got to be a good thing.

 

[fiestaseb]

My turbos at the side of my engine bay and gets rather warm! I'm after a practical set of vents, and considering raising my bonnet

It's a mk5 fiesta

Here's a picture

 

[PumaNoob]

The issue I had with vents was that when it rained, the engine bay was soaked, there was water pooling everywhere an some of the vital plugs induced water and either lost connection due to corrosion, or the water caused them to short giving unwanted extra resistance. I'm sure if you set them up properly with catch trays and drains there wouldn't be any issues

 

[PumaNoob]

Also with regards to the comment about the spacers not doing anything because of the high pressure area above the bonnet, I totally get the theory there, makes a lot of sense, but here is my opposing argument: firstly, the under bonnet temps in my car are highest when stationary , because there is no circulating air. So there is no pressure differential while stationary. But more importantly, if you put a puma in a wind tunnel (or most cars for that matter), there will be a high pressure area on the front and going over the bonnet, but right at the bottom of the windscreen, will be a low pressure area, more so with the bonnet raised, because the flow of air will not go into a corner, it will curve around an internal corner, causing a vacuum of low pressure air, pulling the hot air up out. If you look at the back of a puma bonnet there is a ramp to reinforce this. And because the hot air is less dense, it may actually create a slightly lower pressure going over the screen and top of car, potentially (& very marginally) creating less drag.

Of course this is just my theory, I could be way off the mark on that.

 

[fiestaseb]

To be honest, I'd rather raise the bonnet, in the slightest chance it would drop temperatures, your not really loosing anything?

Cheers pumanoob I didnt take fully into consideration about the vents and drip trays

 

[PumaNoob]

I think I'm going to have to do some tests in this because this thread got me interested, so I started googling, and the general consensus is that at idle and very low speed it lowers temps, but at high speed, it can even Increase under bonnet temps. I can only say for sure that it lowers temps while in traffic or stationary. But I've never checked under bonnet temps at speed. I have the fiesta ghia temp clock, I can move the sensor to the back of the engine bay and see what the temps are like with the spacers and without. But this certainly has my interest now!

 

[CherryVimto]

I think I'm going to have to do some tests in this because this thread got me interested, so I started googling, and the general consensus is that at idle and very low speed it lowers temps, but at high speed, it can even Increase under bonnet temps. I can only say for sure that it lowers temps while in traffic or stationary. But I've never checked under bonnet temps at speed. I have the fiesta ghia temp clock, I can move the sensor to the back of the engine bay and see what the temps are like with the spacers and without. But this certainly has my interest now!

I'd be interested in the results of that's as I'd planned on putting spacers under my bonnet.

 

[Wild E. Coyote]

Also with regards to the comment about the spacers not doing anything because of the high pressure area above the bonnet, I totally get the theory there, makes a lot of sense, but here is my opposing argument: firstly, the under bonnet temps in my car are highest when stationary , because there is no circulating air. So there is no pressure differential while stationary. But more importantly, if you put a puma in a wind tunnel (or most cars for that matter), there will be a high pressure area on the front and going over the bonnet, but right at the bottom of the windscreen, will be a low pressure area, more so with the bonnet raised, because the flow of air will not go into a corner, it will curve around an internal corner, causing a vacuum of low pressure air, pulling the hot air up out. If you look at the back of a puma bonnet there is a ramp to reinforce this. And because the hot air is less dense, it may actually create a slightly lower pressure going over the screen and top of car, potentially (& very marginally) creating less drag.

Of course this is just my theory, I could be way off the mark on that.

You are getting warm there. As the air is going fast over the bodywork it creates the low pressure area. This is what is causing lift and instability of a car at high speeds unless treated correctly (difusor, spoliers, gurney flaps etc). So what you have here is the following: the air over the bonnet is faster than under it; that speed difference creates the pressure differences and the lower pressure is where the air is faster (that is the principle of Venturi effect that is sucking the fuel in carburettor).
For the very same reason is the outlet on F1 cars roughly 1/3 of the inlet size to accelerate the air through the radiators.
Instead of cutting the bonnet and making openings on it, may I suggest something else to bring the air temperatures under the bonnet down? The air has to be let outside, but not necessary at the top of the engine bay.
If you take a look at the clio RS you will see the openings on its wings. Same is for some superchargered Jags. The principle is the same; air is accelerated along the bodywork and that is where is low pressure that would suck the hot air out of engine bay.
I guess it wouldn't be so easy to make openings in wings, but it could be done. However, the top opening (like with raisers) is more effective when the car is stationary as the hot air goes up. The side outlets will be effective only when car is in motion....
Oh, yes, needless to say the FRP wings are easier to be modified for that since they are wider

 

[alexwilliams840]

Turned into quite a topic I was looking at the profile of the Puma and thought because it's quite sleek and with the bonnet raised it shouldn't be high pressure like Pumanoob says... It would be great if we had a wind tunnel, could get a definite answer for the Puma then. A few runs with a temp sensor should give evidence also. I may try that as well at different speeds and see what is revealed

 

[Lord B]

smoke pellets under bonnet, and get someone to film what happens, as you drive past?